A wind turbine is an energy converting device that converts kinetic energy in the wind into electrical energy for use by customers connected to a utility power grid. This type of energy conversion typically involves using wind to turn turbine blades that, in turn rotates the rotor of an AC electrical generator either directly or through a gearbox.
The electrical power available from a wind driven generator and supplied to a utility grid is a function of the power available from the wind, its speed, losses in the grid and the characteristics of the distribution system and loads connected thereto. Because the wind speed and load fluctuates, voltage levels in the grid vary. Likewise since most electric power transmission components have a significant reactive component; voltages in the grid are also a function of the reactive characteristics of loads and components connected to the grid.
To prevent damage to equipment, grid voltage must be held within certain tolerances. One method of supporting grid voltage control is the use of suppliers or absorbers of variable amounts of reactive power to compensate for voltage changes due to the reactive nature of the grid. When overhead lines are primarily inductive, for example, a passive device such as an inductor absorbs reactive power and tends to lower grid voltages while a capacitor supplies reactive power which tends to raise grid voltages
The primary electrical output of an AC generator is from its stator. The output from the stator can be directly connected to the grid or pass through a power converter. One common generator of prior art systems is the doubly-fed induction generator (DFIG) where the output from the stator is controlled by the current in its rotor. The stator in such a system can be directly connected to the grid because the stator voltage and frequency, being controlled by the rotor, can be forced to match the grid voltage and frequency.
A DFIG can also be used as a supplier or absorber of reactive power and therefore contribute to voltage control. The state of the machine depends on whether the level of rotor current provided is greater or less than that needed to provide sufficient flux to generate rated output voltage. When excess current is applied to the rotor of a DFIG the machine is considered to be overexcited. In this state more flux than is necessary is generated by rotor current, the machine supplies or generates reactive power from the stator. By convention, the reactive power from the generator is considered to be positive (flowing from the generator) and typically labeled “+Q”.
If on the other hand the machine receives too little rotor current it is considered to be under excited. In this state the machine absorbs reactive power into the stator to help supply flux. By convention, the reactive power is considered to be negative (flowing into the generator) and typically labeled “−Q”.
A non-DFIG such as a synchronous generator or cage induction machine can also be used as the electrical generator in a wind turbine system that provides controlled reactive power. When these machine types are used in a variable speed configuration a full converter is needed between the stator output and a utility grid. Modern full converters used in wind turbine applications utilize self commutated devices which permit control of the ratio of real to reactive power as well as control of reactive power substantially independent of real power. Prior art systems disclosed in U.S. Pat. Nos. 5,083,039 and 5,225,712 are examples of wind turbines that use a full converter and disclose power factor or reactive power control. Likewise for these systems the direction of reactive power flow (+Q or −Q) is based on the phase relationship between output voltage and current but the conventions regarding the sign of Q is the same as for generators; +Q indicates that the converter is supplying reactive power and −Q indicates that it is absorbing reactive power.
In some prior art systems, when two or more turbines are used in a wind farm a control system determines a particular reactive power for the wind farm or park that is based on either a sensed voltage or a system operator command or both. In this case each wind turbine provides substantially the same amount of reactive power. That is the total required reactive power divided by the number of turbines provides the reactive power support.
Alternatively each wind turbine in a park can adjust its reactive power output based on the same sensed voltage (which is transmitted to each turbine) without an intermediate conversion to a reactive power level for the park. An example is the prior art turbine disclosed in U.S. Pat. No. 6,965,174. As disclosed therein, the turbine uses a control system that linearly increases and decreases a phase angle between real and reactive output from each individual wind turbine once the sensed voltage is outside of a deadband.
With the increasing use of wind turbine generated electrical power, the amount of real power and reactive power provided by wind turbines has increased and the turbine's role in the control of grid operations has become of greater significance. Moreover with the ever increasing number of wind turbines grouped in wind parks, having each individual turbine controlling its reactive power in parallel with all others can lead to undesirable control system interactions. Prior art systems such as disclosed in U.S. Pat. Nos. 5,083,039, 5,225,712, 6,137,187 and 4,994,684 disclose arrangements which control reactive power but do so on a per wind turbine basis.
According to embodiments of the present invention reactive power control is provided by the on-off control of a pre-stored value based on an overall reactive power capability of a cluster of turbines. This approach simplifies wind park design and reduces the load on the SCADA data transmission system. This reduced load results from the elimination of a need to continuously communicate to each turbine a reactive power requirement or a sample of the voltage to be controlled.